Evaluation of Planetary Rover Wheel Performance on Sloped Loose Soil Based on Discrete Element Method (離散要素法を用いた惑星探査ローバー車輪の軟弱斜面走 行性能の評価)

Kurzfassung

Explorations of celestial bodies have been conducted by wheel type robots known as rovers, in recent decades. These planetary surfaces like the Moon or Mars, are covered by fine grain soil, and it is difficult for rovers to explore such environments since rovers slip and get stuck. In fact, NASA/JPL’s Mars Exploration Rover, known as Spirit which landed on Mars in 2004, was buried in loose Martian soil in 2009. In addition, future missions need to account for challenging terrains to explore interesting sites such as lava tubes. Thus, it is an essential task for lunar/planetary rovers to improve their traveling performance. Therefore, the objectives of this thesis are to investigate the influence of the grouser wheel parameters on the wheel performance and to design the appropriate wheel for future exploration rovers. Researchers have studied the wheel performance based on both simulation and experiment so far. In particular, numerical simulations are widely used in the field, thanks to the improved computational hardware in recent years. As one of the commonly used numerical methods in wheel-soil contact simulation that include soil deformation, there is the Discrete Element Method (DEM). The DEM is a particle-scale numerical method and can handle the discontinuity model. The advantages of this method are capturing many features of the soil dynamics behavior and being able to consider the soil deformation. On the other hand, for this research work, the studies cover the performance of wheel with grousers under fine particle environments. The observation and simulation of wheel-soil interaction are key elements in this research. Hence, DEM is applied for this study to observe the wheel-soil interaction on particle levels. As a result of this research, the developed DEM simulation model is first validated through a series of experiments, including angle of repose tests and single wheel tests. Furthermore, the parametric wheel simulation is studied by changing several key wheel parameters through a reiterative process for designing a wheel. As a result, the relationship between the soil behavior and the wheel performance is understood through this research. Finally, the appropriate wheel is proposed by combining less slippage and less sinkage based on the parametric simulations. This proposed wheel can indeed reduce the wheel slippage and sinkage on loose soil. These results show that rovers with proposed wheels could be able to explore on finegrain soil more easily and travel more challenging environments like sloped terrain required for future exploration missions. Moreover, the outcomes have a potential to develop better wheel for Mars exploration rovers.